For years, one of the major problems associated with using ceramic materials for high strength applications has been the inability to sinter the materials to theoretical density without the need for high temperatures and pressures. Pores remaining in the microstructure reduce the strength of the fired ceramic. In some of the first work of its kind, Rhodes showed a relationship between agglomerate size and sintering of yttria-stabilized zirconia for powders having an average crystallite size of approximately 10 nm (Rhodes, W. H. J. Am. Ceram. Soc. 1981, 64(1), 151-170). By eliminating agglomerates through sedimentation, he was able to lower the sintering temperature 300.degree. C. for powders of identical crystallite size. This type of behavior has been demonstrated in other systems as well (See, for example, Tremper, R. T. and Gordon, R. S. in "Agglomeration Effects on the Sintering of Alumina Powders Prepared by Autoclaving Aluminum Metal;" Ceramic Processing Before Firing, Onoda, G. Y. and Hench, L. L. (Eds.); John Wiley and Sons, New York (1978) pp. 153-175; and Sordelet, D. J. and Akino, M. J. Am. Ceram. Soc. 1988, 71(12), 1148-1153).
In an attempt to improve the sintering characteristics of ceramic powders, chemical techniques to synthesize submicron particles have been widely investigated. One of these techniques is precipitation from homogeneous solution (PFHS). This method can provide uniform nucleation, growth and aging of the particles throughout the solution. Particle morphology and size can be controlled by changing different reaction parameters (ion concentrations, temperature, aging time, etc.). This technique is usually inexpensive and relies on simple benchtop chemistry to synthesize "ideal" (submicron and monosized) precursor particles.
In most cases, precipitated powders must be calcined to the oxide before processing can be initiated. In some cases, the smaller more reactive particles form solid aggregates due to surface area reduction driven initial stage sintering (for additional discussion of this process, the reader is referred to Lange, F. F. and Miller, K. T. J. Am. Ceram. Soc. 1987, 70(12), 896-900). When this aggregation occurs, advantages gained by controlled synthesis of the "ideal" particles are lost.
Chain aggregation during nucleation and growth of a particular phase has also been observed during heat treatment of freeze-dried aluminum sulfate (See, Johnson, D. W. and Schnettler, F. J. J. Am. Ceram. Soc. 1970, 53(8), 440-444). In the work of Johnson and Schnettler, very large 300-400 micron diameter frozen spheres of aluminum and iron sulfates were prepared. Within these amorphous sulfate droplets, chained aggregates of crystallites formed upon heating in a pattern established during the freeze-drying step. The authors state that the initial gamma-Al.sub.2 O.sub.3 is transformed to the alpha phase, and, after the transformation, the alpha-Al.sub.2 O.sub.3 nucleates and grows to provide the chained aggregates. This study involved chain aggregation within nearly millimeter-sized spheres and does not involve the interparticle association of dispersed micron- and submicron-sized lead oxide precursors.
In another study reported by Hirayama, T. in J. Am. Ceram. Soc. 1987, 70(6), C122-C124, a specially synthesized experimental transition Al.sub.2 O.sub.3 powder with ultrafine spherical particles was found to have an alpha phase transition temperature of 1335.degree. C. compared with the transformation temperature of 1280.degree. C. observed for commercial samples. The author of this article noted that particle coarsening never occurred for the experimental Al.sub.2 O.sub.3 particle until it transformed to the alpha phase, whereas, for the commercial samples, particle coarsening always preceded the transformation to the alpha phase. The material described in this article was unique, even for Al.sub.2 O.sub.3 powders, and it behaved quite differently than readily available materials. The notion that particle coarsening occurs only at the alpha phase transition cannot be applied generally to all aluminum oxide preparations let alone to other oxide preparations composed of a transition metal component which is altogether distinct from aluminum.